The invention concerns a hybrid drive for a motor vehicle in accordance with a generic concept thereof.
A drive system for a motor vehicle normally possesses at least one electrical machine which itself, for instance, can comprise the drive parts.
Electric motors, as drive motors, which are furnished with electrical energy from a battery or an on-board fuel combustion cell, make emission free and nearly noiseless travel possible. An added advantage of such motors is that they are compact in size. As a disadvantage, the travel capacity of a purely electrically driven vehicle is considerably limited because of the low storage capacity of available batteries. On this account, up to now, electric vehicles only find application in special areas.
Besides purely electrical drive systems, partially-electric vehicle drives are also known which, in the practice, are designated as xe2x80x9cHybrid Drivexe2x80x9d vehicles. These partial electric drive systems mainly possess an internal combustion motor, as a drive aggregate, which makes a large power capacity possible and extends the travel radius of the motor vehicle. To further implement this advantage, at least one electric machine is placed between the internal combustion motor and the transmission. The said electric machine is placed in series or in parallel with the input gear train and simultaneously offers the advantages of an electric drive, such as induction braking and an emission free travel.
In the case of serial-hybrids, in a travel area with limited demand, the operation can be purely electric, that is, the internal combustion motor and one electric motor are idle, wherein the entire amount of needed energy is furnished by a battery. In a second travel area, where the travel is made via the internal combustion motor which also drives the electrical machine, now operating as a generator, which rotation delivers the energy for the driving motor during electric operation. Such serial hybrid drives operate without constant clutch engagement.
In the case of parallel hybrids, where an electric motor is connected to the input shaft of the transmission, starting motion from rest can be carried out alone with an electric drive. During this electric run, the internal combustion motor idles and is separated from the transmission input shaft by a constant clutch engagement. Upon a call for greater capacity, for instance, to reach a definite high driving speed, the said internal combustion motor is started by the closing of the clutch whereby said internal combustion motor then serves as the primary source of input power. The electric motor can then be used as an auxiliary power source, a booster or as a generator for charging the vehicle battery.
A parallel-hybrid of this kind, with an internal combustion motor, an automatic transmission, a battery, an alternating converter, and a permanently activated synchronizer is, for example, described in the German publication xe2x80x9cVDI-Berichtexe2x80x9d (Assocation of German Engineers Report), No.: 1378, (1988), M. Lehna: xe2x80x9cAudio-duo, a hybrid vehicle for City Logisticsxe2x80x9d, pages 119 to b 128.
In this case, the disadvantage of these known parallel-hybrids is, however, that up to the closure of the coupling, a noticeable traction power interruption occurs in the take-off shaft.
With a conventional starter, the internal combustion motor is brought up to its starting rotational speed, in that the starter torque is transferred by a pinion gear to a flywheel of the motor. These known starters are flanged onto the motor block in such a fashion that upon activation, the pinion gear meshes with a ring gear on the flywheel in order to set the motor into rotation.
In the case of such starters, a further disadvantage arises in that their power is not sufficient and their operational life is very limited.
The limited length of operational life makes these starters inappropriate for on-off commands to the internal combustion motor, i.e. the xe2x80x9cStart-Stop-Operationxe2x80x9d wherein parallel-hybrids with an electric motor adapt themselves fundamentally for such a mode of operation as would noticeably reduce the use of fuel and in which the internal combustion motor would shut off instead of idling and only start when it was really needed.
From the April 1988 issue of the xe2x80x9cEuropean Automotive Designxe2x80x9d publication, page 24, an alternative starter unit is made known in which an electric motor, installed between the internal combustion motor and the transmission, is directly connected to the drive shaft of the said internal combustion motor and, in addition to its starter function, can assume the function of a generator.
In the case of this development, nevertheless, it could not be avoided that a traction break occurred at the coupling of the crankshaft of the internal combustion motor and the input shaft of the transmission.
A further method, known in the practice, of accelerating the internal combustion motor to its starting rotational speed, that is, to its idling speed, arranges to couple a preliminary start of the internal combustion motor with the transmission input shaft, while acceleration from rest is carried out by the electric motor. The clutch, in this case, does not engage until the difference of the rotational speed is zero.
Where only a single electric motor stands available for this purpose then, when motionless, the vehicle must first start the internal combustion motor, while the transmission is in neutral and the clutch is closed. During this time, the vehicle cannot be driven. Only when the internal combustion motor is up to speed can the clutch, between the internal combustion motor and the transmission, be placed in first gear, whereupon, the normal process of getting underway begins.
Disadvantageously, the start of the vehicle is marked by a clear time delay by this procedure.
A further disadvantage of the known hybrid-drive is found therein in that the hydraulic pump for the transmission which, for example, is integrated into the said transmission, is difficult to bring up to the necessary rotational speed to produce the needed pressure for the shifting elements when the vehicle is starting from rest. The reason for this is that the pump is usually driven by the internal combustion motor, which is not running at that moment due to the starting operation of the electric motor from rest.
In practice, this problem is solved by an external placement of the hydraulic pump and a drive therefor from a dedicated energy source, which shows itself as very expensive and complex.
The present invention has the purpose of creating a hybrid drive for a motor vehicle in which the current supply of the said vehicle and its aggregate is efficient and by which, in a constructively simple manner, a purely electrical operation with a small need for capacity can be realized and a power input of an internal combustion motor can be achieved at an increased demand for load, during which a change of gear is avoided as far as possible.
In accord with the invention, this purpose is achieved by a hybrid drive according to the invention described in further detail below.
In the case of the hybrid drive in accord with the invention, a very economical and environmentally friendly, purely electrically operated start from rest or from low vehicle speeds is possible in an advantageous manner, such starts that exist, for example, in a stop-go operation or during in-and-out parking procedures. During closure of the second clutch, between the internal combustion motor and the electrical machine, the already running internal combustion motor is engaged so that full vehicle power is available.
By means of the invented placement of the electric machines and the clutches, the gear-change function can be carried out without a significant traction detriment coupling of the internal combustion motor and the input transmission shaft occurs at a small difference in rotational speeds which, in the optimal case, is practically zero.
By means of the invented interposing of a second electric machine between the internal combustion motor and the input shaft of the transmission of the input drive train, a first electric machine can advantageously take on a plurality of functions. That first electric motor can serve as a starter for the internal combustion motor or as a generator for the electrical system, the latter including charging the battery or for the delivery of electrical power to other user operations, such as a drive for the hydraulic pump of the vehicle transmission and probably as a drive for further subordinate aggregates such as an air conditioner compressor, a power steering pump, a braking amplifier, or the water pump.
Hence, the invention offers the advantage that a multiplicity of electrical components, such as drives for the starter, the generator, or the pump become superfluous which, in the case of conventional hybrid drives, are substantial cost factors.
Further, with the invented achievements of the purposes of the invention, an improved start-stop operation is possible since the internal combustion motor, during a purely electrical starting up routine, can be started by the first electrical machine in a short time without a reactive effect on the crankshaft.
An additional advantage is a reaction improvement when starting from rest with a high traction requirement. If, when in the case of a start from rest with purely electrical means is underway and a high traction need suddenly occurs, for instance, on a mountain or during the need for a forced downshift, then the internal combustion motor must be automatically started. With the invented arrangement in which a second electrical machine can be in use for acceleration from rest or low speed and, at the same time, the internal combustion motor can be started by a first electrical machine and engaged by means of a slip clutch, a substantial improvement in the reaction time can be achieved.
This operation advantageously provides a large availability of traction which comprises the sum of the torque of the internal combustion motor, the torque of the first electrical machine, and the torque of the second electrical machine.
The invention is particularly adapted for an automatic drive, although it fits well in any type of transmission.